This invention relates to a data exchange for exchanging an input data block into an output data block to carry out transmission of the output data block.
It is to be noted throughout the instant specification that a data exchange is for use in an asynchronous transfer mode (ATM) communication system and may therefore be called a cell data exchange for exchanging an input data block into an output data block although this invention may not be restricted to the cell data exchange. The cell data exchange comprises first and second exchanging sections each of which exchanges an input cell data block into an output cell data block. One of the first and the second exchanging sections serves as an active exchanging section. Another one of the first and the second exchanging sections serves as a stand-by exchanging section. It is well known in the art that the first and the second exchanging sections have first and second buffers, respectively. When the first exchanging section is operated as the active exchanging section, the first buffer memorizes a plurality of input cell data blocks as memorized cell data blocks. Each of the memorized cell data blocks is read as the output cell data block out of the first buffer. On the other hand, the second buffer is empty during operation of the first buffer.
A conventional cell data exchange further comprises a receiving section and a transmitting section. The receiving section is operable to receive a received cell data block and to supply the received cell data block as the input cell data block to the active exchanging section, for example, the first exchanging section. The transmitting section is for carrying out transmission of the output cell data block.
It is assumed that the first exchanging section serves as the active exchanging section and the second exchanging section serves as the stand-by exchanging section, as mentioned above. Under the circumstances, let the first exchanging section be often changed to the stand-by exchanging section according to a result of a situation of the first exchanging section under control of a control unit in the following manner. In this event, the receiving section at first instructs to the preceding stage equipment not to send any more cell data block by sending the control signal. At the same time, the first exchanging section stops producing the output cell data block. As a result, some memorized cell data blocks are held as residual cell data blocks in the first buffer. Then, the residual cell data blocks are transferred to the second buffer through a transfer path to form the second buffer having the same contents as the first buffer. After completion of transferring the residual cell data blocks from the first buffer to the second buffer, the first exchanging section is changed over to the stand-by exchanging section and the second exchanging section is concurrently changed over to the active exchanging section. Finally, the control signal is sent to the preceding stage equipment to restore the previous instruction and to instruct that the change over operation has been completed and the receiving input cell data block is now ready.
However, the change over operation described above in the conventional cell data exchange has the following several disadvantages. The first is that the change over operation causes delays in transmission of cell data blocks due to transferring the residual cell data blocks held in the first buffer to the second buffer, and this is not suitable for the application of the communication requiring strict real time operation. The second is that the change over operation requires relatively complicated control procedures. The third is that traffic congestion occurs in the preceding stage equipment due to the control signal which has been instructed to stop sending cell data blocks temporarily during the change over operation. This may cause loss of some input cell data blocks. The fourth is that a large capacity and high speed transfer path is required in a large system to transfer the residual cell data blocks from the first exchanging section to the second exchanging section.